验证Hashmap不支持同步，ConcurrentHashMap支持_JAVA

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验证Hashmap不支持同步，ConcurrentHashMap支持

2014/8/15 15:34:45 xmind 程序员俱乐部我要评论(0)

摘要：一直都不知道concurrenthashmap有什么实际的用处？先写个例子比较下hashmap和它。方法用2000个线程下同一个key值，同步的话，应该最后的map的size为1，不同步可以大于1.JavaCode12345678910111213141516171819202122232425262728293031publicclassHashMapSyn{publicstaticvoidmain(String[]args)throwsInterruptedException{System
标签：has Map Hash 同步

一直都不知道concurrenthashmap有什么实际的用处？先写个例子比较下hashmap和它。
方法用2000个线程下同一个key值，同步的话，应该最后的map的size为1，不同步可以大于1.
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31 ? public?class?HashMapSyn?{
????public?static?void?main(String[]?args)?throws?InterruptedException?{
????????System.out.println(runMapPutAndSizeShouldBe1(new?HashMap<String,?String>(32)));
????????System.out.println(runMapPutAndSizeShouldBe1(new?ConcurrentHashMap<String,?String>(32)));
????}
????
????public?static?int?runMapPutAndSizeShouldBe1(final?Map<String,?String>?map)?throws?InterruptedException?{
????????Thread?t?=?new?Thread(new?Runnable()?{
????????????@Override
????????????public?void?run()?{
????????????????for?(int?i?=?0;?i?<?2000;?i++)?{
????????????????????new?Thread(new?Runnable()?{
????????????????????????@Override
????????????????????????public?void?run()?{
????????????????????????????map.put("1",?"1");
????????????????????????}
????????????????????},?"Thread?inside?"?+?i).start();
????????????????}
????????????}
????????},?"Thread?outside");
????????t.start();
????????t.join();
????????return?map.size();
????}

????@Test
????public?void?testHashMapNotSynchronize()?throws?InterruptedException?{
????????Assert.assertEquals(1,?runMapPutAndSizeShouldBe1(new?ConcurrentHashMap<String,?String>(32)));
????????Assert.assertNotSame(1,?runMapPutAndSizeShouldBe1(new?HashMap<String,?String>(32)));
????}
}
当然，hashmap的最后可能测不出来，可以将hashmap拷到本来加Thread.sleep。
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22 ? ????public?V?put(K?key,?V?value)?{
????????if?(key?==?null)
????????????return?putForNullKey(value);
????????int?hash?=?hash(key.hashCode());
????????int?i?=?indexFor(hash,?table.length);
????????for?(Entry<K,V>?e?=?table[i];?e?!=?null;?e?=?e.next)?{
????????????Object?k;
????????????if?(e.hash?==?hash?&&?((k?=?e.key)?==?key?||?key.equals(k)))?{
????????????????V?oldValue?=?e.value;
????????????????e.value?=?value;
????????????????e.recordAccess(this);
????????????????return?oldValue;
????????????}
????????}
????????modCount++;
????????
????????/**?add?sleep?for?test*/
????????try?{?Thread.sleep(1);?}?catch?(InterruptedException?e1)?{}
????????
????????addEntry(hash,?key,?value,?i);
????????return?null;
????}
?